Understanding List Controls

I have a simple list of the kind below and am trying to get the left and right buttons to behave as  expected.

http://ix.io/1BXn

In desperation I have right adding <onright>82</onright> and  <onleft>81</onleft> to no avail. I have tried looking at other skins but can't quite seem to figure out how these attributes are supposed to work.